1. Field of the Invention
The present invention relates to an apparatus and method adapted to control the brightness of a display such as a liquid crystal display (LCD) in a portable computer.
2. Background of the Related Art
Products directly using an LCD as a display unit include a desktop computer and various portable appliances such as a notebook computer and a personal digital assistant (PDA). Portable appliances such as a notebook computer and a PDA have an important object to reduce or minimize the consumption of electric power.
FIG. 1 schematically illustrates the configuration of a related art portable computer, for example, a notebook computer. As shown in FIG. 1, a notebook computer includes a central processing unit (CPU) 10, a video controller 11, a host-PCI bridge 12, a memory 13, a video RAM 14, an audio controller 15, a LAN controller 16, a card bus controller 17, a PCI-ISA bridge 18, an LCD 19, a microcomputer (micom) 20, and a keyboard 21, all of which are connected by bus lines.
The PCI-ISA bridge 18 includes a CMOS-RAM 180. The microcomputer 20 includes a ROM 200, a RAM 201, and a keyboard controller 203.
As shown in FIG. 2, the LCD 19 is provided with a light emitting element 190 such as a cold cathode fluorescent lamp (CCFL) at a lower or upper portion thereof. The notebook computer also includes a unit for controlling the brightness of the LCD 19. The brightness controlling unit includes a power supply unit 30 and an inverter 33. The power supply unit 30 is for transforming a voltage supplied from a battery 31 or an AC adapter 32 into a predetermined level, and supplying the voltage of the predetermined level. The inverter 33 is for converting the voltage of the predetermined level supplied from the power supply unit 30 into a signal having a waveform synchronized to a PWM signal, and applying the converted signal to the CCFL 190.
The PWM signal, which is inputted to the inverter 33 from the microcomputer 20, may be set to have a fixed frequency from 100 Hz to 400 Hz, for example, 210 Hz. In this case, the inverter 33 outputs a signal having a waveform synchronized to the frequency of 210 Hz. The output signal from the inverter 33 is applied to the CCFL 190 of the LCD 19, so that the brightness of the LCD 19 is maintained at a certain level. For brightness control, the inverter 33 receives information about the on-time duty at the selected frequency, adjusted in a range of 0 to 100% in accordance with a desired level of brightness.
Thus, the inverter 33 converts the predetermined voltage level supplied from the power supply unit 30 into a signal having a frequency and on-time duty synchronized to the PWM signal outputted from the microcomputer 20, and outputs the resultant signal to the CCFL 190 to control the brightness of the LCD 19. In operation, however, the frequency of the signal outputted from the inverter 33 in sync with the PWM signal may interfere with the frame frequency of the LCD 19 so that noise appears on the screen of the LCD.
Accordingly, the PWM frequency of the inverter is generally set based upon the frame frequency of the LCD. Typically, the PWM frequency is set to be higher than the n-th multiple (n times) of the frame frequency, that is, a vertical sync (Vsync) frequency, by 20 to 30 Hz. If the difference between the PWM frequency and the n-multiple of the frame frequency is less than 20 Hz, the possibility increases that noise appears on the LCD because of frequency interference.
The noise generation occurrence caused by frequency interference can be represented by an expression of “f=ABS[PWM Frequency−(Frame Frequency×n)] (where n=1, 2, 3, 4, . . . ). In this expression, “f≧15” corresponds to a stable state, and “f<15” corresponds to an instable state.
Accordingly, where the frame frequency of the LCD, that is, the Vsync frequency, is 60 Hz, appropriate PWM frequency ranges may be as follows: (60*1)+20˜30=80˜90; (60*2)+20˜30=140˜150; (60*3)+20˜30=200˜210; (60*4)+20˜30=260˜270; (60*5)+20˜30=320˜330; and (60*6)+20˜30=380˜390. Respective central frequency values of these frequencies, that is, 90 Hz, 150 Hz, 210 Hz, 270 Hz, . . . may be used as optimal setting values for a PWM frequency. In particular, 210 Hz or 270 Hz are used as a PWM frequency. With respect to a central frequency value of 270 Hz, a frequency range of 255 Hz to 285 Hz can be considered a stable PWM frequency range for the LCD with a frame frequency of 60 Hz. Where an LCD using a single fixed Vsync frequency, for example, an LCD in which only a Vsync frequency of 60 Hz is allowed for the same kind (e.g., model) of portable computers, its PWM frequency may be set in accordance with the above-described manner.
However, the related art apparatus and methods for controlling brightness of an LCD, for example, in a portable computer have various disadvantages. Where different LCDs using different Vsync frequencies of, for example, 50 Hz, 56 Hz, and 60 Hz (or 45 Hz, 57 Hz and 60 Hz), are used for portable computers of the same model, it is difficult or impossible to select a PWM frequency for all the LCDs. As a result, noise may be generated in a particular one of the LCDs. For example, where the PWM frequency is fixed at 210 Hz to meet a Vsync frequency of 60 Hz, the LCDs using a Vsync frequency of 50 Hz or 56 Hz involve generation of noise caused by frequency interference because the difference between a multiple of the Vsync frequency and the PWM frequency of 210 Hz corresponds to 10 Hz or 14 Hz (f<20). Thus, when related art notebook computers are configured to control the frame frequency of an LCD, degradation in picture quality can occur because of interference between the frame frequency of the LCD and the PWM frequency for controlling the brightness of the LCD.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.